Lecture 4: Origin of Earth’s Volatiles

Abiol 574

“Excess volatiles”• Term coined by

William Rubey (circa 1955)

• Definition: Compounds present at Earth’s surface that were not derived from converting igneous rock to sedimentary rock

• Rubey and other geologists presumed that the atmosphere and oceans were derived from outgassing by volcanoes

H2O is one important excess volatile Others include CO2, N2, S, and Cl

Impact degassing• We now think that

many of Earth’s volatiles, including water, may have been released directly to the surface by impacts

• Large impacts are predicted by models of planetary accretion

• The process of volatile release during impacts is called impact degassing

• If Earth’s atmosphere was predominantly formed from impacts, we can learn more about it by looking at meteorites..

Two basic types of meteorites

http://www.cpither.freeserve.co.uk/sporadic_meteors.htm

• Made of (you guessed it..)

• Made of silicates

Iron meteorites

http://www.cpither.freeserve.co.uk/sporadic_meteors.htm

• These objects formed when the differentiated cores of large planetesimals were subsequently disrupted by collisions

Ordinary chondrites

• Ordinary chondrites are a type of stoney meteorite that (usually) contain chondrules

• Definition: chondrules—millimeter-sized inclusions in some meteorites that formed (somehow) within the solar nebula

Ref.: J. K. Beatty et al., The New Solar System, Ch. 26

Stoney meteorite classifications

Ref.: J. K. Beatty et al., The New Solar System, Ch. 26

Carbonaceous chondrites

Ref.: J. K. Beatty et al., The New Solar System, Ch. 26

ALH77307

Compositions fromAllende meteorite

• CI Carbonaceous chondrites are considered to be the most similar in composition to the solar nebula• They do not have chondrules!

Volatiles in meteorites

• Carbonaceous chondrites are rich in water and other volatiles– Up to 20 wt.% H2O (although some of this

may be absorbed by the meteorite after it hits the Earth)

– Approximately 3.5 wt% organic C– Nitrogen and noble gases are trapped within

the organic carbon matrix• Ordinary chondrites are much less

volatile-rich– Roughly 0.1 wt% H2O

Is the Earth formed from chondrites?

• Mass of Earth: 61024 kg• Mass of oceans: 1.41021 kg• Ordinary chondritic planet:

61024 kg (0.001) = 61021 kg = 4 oceans

• Carbonaceous chondritic planet: 61024 kg (0.15) = 91023 kg

= 600 oceans!– So, we only need a few carbonaceous-type

planetesimals to get Earth’s water– Alternatively, we could build the Earth from ordinary

chondrites. But then we run into the problems mentioned last time, i.e., it is difficult to form hydrated silicates in the inner Solar System

Asteroid belt

• Range: 2-3.5 AU– Mars: 1.5 AU– Jupiter: 5.2 AU

• Inner belt (2-2.5 AU)– “S-type” asteroids

• Outer belt (2.5-3.5 AU)– “C-type” asteroids– These ones are

thought to be carbon-rich, like carbonaceous chondrites

http://content.answers.com/main/content/wp/en/thumb/8/80/350px-InnerSolarSystem-en.png

Kirkwood Gaps

http://en.wikipedia.org/wiki/Kirkwood_gap

• So, could water-rich planetesimals from the outer asteroid belt region have hit the Earth during accretion?

Yes!

Accretion of volatiles

Raymond et al., Icarus (2006)

• Yes, it is possible for planetesimals to migrate in from the outer asteroid belt region during accretion• The planet formed at 1 AU in this particular simulation is extremely water-rich: oceans would be 10’s of kilometers deep!

Stochastic volatile delivery

Raymond et al., Icarus (2004)

• Outcomes of 11 different simulations• Some planets formed near 1 AU are wet, others are dry

SolarSystem

• Another way to approach the problem of delivery of volatiles/formation of the atmosphere and oceans is to use noble gases

• Why?

• The noble gases occupy the rightmost column of the periodic table• Their outer shell of electrons is completely filled

• Another way to approach the problem of delivery of volatiles/formation of the atmosphere and oceans is to use noble gases

• Why?

• Answer: Because they are chemically unreactive (except possibly for Xe) and, hence, they should just tend to sit in a planet’s atmosphere, if they don’t escape

Solar noble gases (non-radiogenic)

Ref: H. D. Holland, Chemical Evolution of the Atmosphere and Oceans (1984), p. 33. (After Anders and Owen, 1977)

• The lighter noble gases are most abundant in the Sun, and presumably in the solar nebula, as well

Noble gases in Earth’s atmosphere

g/g planetGas Concentration (ppmv) g/g solar

20Ne 16.5 4.510-9

36Ar 31.5 3.910-7

84Kr 0.65 2.910-5

132Xe 0.0234 1.110-4

Ref.: Holland, H. D., The Chemical Evolution of the Atmosphere and Oceans (1984), p. 30

• • But the lighter noble gases are depleted in Earth’s atmosphere relative to solar abundances• What does this tell us?

What does this tell us?

1. Earth’s atmosphere did not form primarily from gravitational capture of gases from the solar nebula

– Or, if there was a captured atmosphere, it must have been nearly entirely lost, perhaps during the Moon-forming impact

2. Whatever process brought in the noble gases delivered the heavy ones more efficiently than the light ones

Planetary noble gas abundances

• Venus has ~100 times more noble gases than Earth, while Mars has ~100 times less

• Venus, Earth, and Mars all have roughly the same pattern of elemental abundances

• Meteorites have more Xe than does Earth (or Venus or Mars) “Missing xenon”

problem

Ref.: T. Owen et al., Nature (1992), Fig. 1

• One can also learn things by looking at isotopic ratios…

Terrestrial xenonisotopes

Ref.: R. O. Pepin, Icarus (1991)

• Linear fractionation pattern could be explained by hydro- dynamic escape of hydrogen dragging off Xe (Pepin, 1991)• Dragging off Xe, however, would entail dragging off everything else• It should also fractionation Kr isotopes very strongly, and this is not observed --So, Pepin assumes that Kr is replaced later, while Xe is not (?)

Neon isotopes• 3-isotope plots can be used

to distinguish gases coming from different sources

• Data shown are neon isotope ratios in MORBs (midocean ridge basalts)

• Earth’s atmosphere is depleted in 20Ne relative to 22Ne– 21Ne is radiogenic and is

simply used to indicate a mantle origin

• Mantle Ne resembles solar Ne– Ne is thought to have been

incorporated by solar wind implantation onto dust grains in the solar nebula

• The atmospheric 20Ne/22Ne ratio can be explained by rapid hydrodynamic escape of hydrogen, which preferentially removed the lighter Ne isotope

Ref: Porcelli and Pepin, in R. M. Canup and K. Righter, eds., Origin of the Earth and Moon (2000), p. 439

Volatiles from comets?• The comet model can

successfully explain the relative ratios of Ar, Kr, and Xe (thereby solving the “missing Xe problem”)– This can be simulated in

the lab by looking at low-temperature adsorption of gases onto amorphous ice

• Terrestrial planets fit on a mixing line between an indigenous source and comets

• Ne has to come in by another route, as mentioned previously

Ref.: Owen et al., Nature (1992), Fig. 2b

D/H Ratios• But the comet model

fails to account for the D/H ratio of the oceans (or, at least, it used to fail…)

• Cometary D/H is at least twice that of seawater (but see below)

• D/H in carbonaceous chondrites is scattered, but its average is close to that of Earth’s oceans

• Could Earth’s noble gases and its water have come from different sources?

Ref: Owen and Bar-Nun, in R. M. Canup and K. Righter, eds., Origin of the Earth and Moon (2000), p. 463

D/H from a Kuiper Belt comet

• Comet Hartley 2 is a short-period comet that appears to have a terrestrial D/H ratio (P. Hartogh et al., Nature, 2011)

• Characteristics– 2.250.6 km– Albedo = 0.028

• Orbital characteristics– P = 6.46 yr– e = 0.694– i = 13.6”

• Passed 0.12 AU from Earth on Oct. 20, 2010

Hartley 2 from EPOXI (formerly DeepImpact)

D/H from a Kuiper Belt comet

Hartogh et al., Nature (2011)

• If this is indeed a former Kuiper Belt object, then we may need to re-evaluate our models for how D/H varied in the early Solar System